1. Field of the Invention
The present invention relates to a power supply system, provided with a current supply system having a high voltage system and a low voltage system, for driving a motor with power from the high voltage system.
2. Description of the Related Art
Conventionally, internal combustion engines (engines) using fuel such as gasoline have been widely used as a source of power for vehicles. Various types of device are installed in this type of vehicle, and a lot of these devices are driven by electrical power. Vehicles are therefore fitted with a battery, and various electrical devices mounted in the vehicle are powered by electrical power from this battery.
Some of these types of vehicle mounted electrical components have comparatively large power consumption, such as motors such as starter motors, air conditioner motors, power steering motors etc., or various heaters. Electrical power of electrical devices is determined by multiplying voltage by current. With large power devices, if the source voltage is low, then a large current must flow to obtain large amounts of power, but the power consumed by comparatively low resistances such as the internal resistance of the battery and wiring can not be ignored, and there is also a large amount of loss in the motor and an inverter driving the motor. There is therefore a demand to make battery voltages high.
Also, hybrid vehicles and the like use a motor for traveling and for engine torque assist. With this type of system, a high voltage battery that efficiently drives such motors is desired.
Generally, in vehicles power is supplied to vehicle mounted components by fitting a battery capable of driving ancillary device loads of about 14V-1 kW, but when using large power devices such as those described above, there is a strong demand to use a higher voltage power source.
It has therefore been proposed to provide a low voltage 12V battery for the low power devices, and a high voltage battery, for example a 36V battery, as a separate battery for the large power devices.
This type of system is constructed as shown, for example, in FIG. 18, having a 36V battery B1 connected via an inverter INV to a motor generator M/G, and also connected to another 42V system load LD1. The 36V battery B1 is also connected via a DCDC converter CONV to a 12V battery B2 and a 14V load LD2.
Accordingly, the 42V load LD1 and the motor generator M/G are driven using high voltage power from the 36 V battery B1, while the generated output of the motor generator M/G is used as charging power for the 36V battery B1.
On the other hand, high voltage power from the 36V battery B1 is converted to 14V by the DCDC converter CONV to charge the 12V battery B2, and then the 14V load LD2 is driven by 14V supply from the 12V battery B2.
There is also a demand to efficiently charge each of the power supplies with this type of 42-14V dual power supply system having both a 36V and a 12V battery. It is also possible to provide two generators respectively corresponding to each power supply, but it is difficult to keep the cost of a system having two generators down. It has therefore been considered to charge one power supply with a voltage generated from one generator, and then charge the other power supply by subjecting this generated voltage to voltage conversion, but in order to do this, a DCDC converter is required.
For example, in a dual power supply system such as that disclosed in Japanese Patent Laid-Open No. 2000-324857, a dedicated DCDC converter circuit is provided for carrying out voltage conversion between the two power supplies. A high voltage of a few hundred volts is converted using this dedicated DCDC converter, to charge a low voltage battery (a 12V battery).
However, by providing the dedicated DCDC converter circuit, the overall cost of the dual power supply system is increased significantly. In particular, in the case where a voltage ratio of two power supplies is not as large as that disclosed in Japanese Patent Laid-Open No. 2000-324857, such as a system provided with a 42V power supply and a 14V power supply, since a requirement with respect to voltage conversion range is not so large, it is desirable to realize a voltage conversion function at as low a cost as possible.
It is an object of the present invention to provide a system with an output voltage ratio of two power supplies that is not too large (for example, low voltage:high voltage=1:2-1:4), such as a 42V-14V type power supply, at low cost.
The present invention is a power supply system provided with a high voltage system and a low voltage system, comprising an a.c. motor that is a load on the high voltage system and is driven by a high voltage power source, an inverter for controlling supply of power to the a.c. motor, a low voltage system supply path for applying a neutral point voltage of the a.c. motor to a load of the low voltage system, and a load of the low voltage system for connecting to the low voltage system supply path, wherein a voltage ratio of the low voltage system to the high voltage system is set to from approximately 1:2 to approximately 1:4.
By having the high voltage system and the low voltage system, it is possible to obtain an appropriate supply voltage according to the load to be driven, to efficiently drive the load. By connected the high voltage system and the low voltage system through the a.c. motor, there is no need for a DCDC converter, which enables simplification of the system and reduction in cost. Further, by having the voltage ratio of the low voltage system to the high voltage system in a range of 1:2 to 1:4, it is also possible to maintain efficient drive of the a.c. motor.
It is also preferable to control power supply to the a.c. motor using an inverter so that the voltage ratio of the low voltage system to the high voltage system is a specified value between 1:2 and 1:4. In this way, it is possible to effectively control neutral point voltage.
It is also possible to make the voltage ratio of the low voltage system and the high voltage system approximately 1:3.
Because the voltage ratio of the low voltage system to the high voltage system is made a specified value between 1:2 and 1:4, it is possible to perform control so that an a.c. component in output from the inverter is not a sine wave shape, but follows a current distortion phenomena.
It is also preferable for the a.c. motor to be a motor for generating vehicle traveling force or a motor for starting an engine.
It is further preferable to have a relay for opening and closing the low voltage supply path, and determining means for determining whether or not inverter control for making the voltage ratio a specified value is appropriate, with the relay being opened if it is determined by the determining means that control is inappropriate.
By providing the relay in this way, the low voltage system is disconnected as required, and it is possible to ensure efficient drive of the a.c. motor.
It is also preferable for the determining means to determine whether or not the a.c. motor attains the required output force.
It is also preferable for the determining means to determine whether or not efficiency of the a.c. motor is as specified or better. p It is still further preferable, when opening or closing the relay, to change a motor output command so as to control a.c. motor output variations attributable to variations in neutral point voltage based on opening and closing of the relay.
It is further preferable, when opening or closing the relay, to open or close the relay after controlling the neutral point voltage to zero.
It is also preferable to have a return diode, connected from the low voltage system to the high voltage system, for returning power from the low voltage system to the high voltage system.
The present invention is also directed to a power supply method in system having an a.c. motor driven by a high voltage power source, and an inverter for controlling supply of power to the a.c. motor, provided with a high voltage system and a low voltage system for connecting a neutral point of the a.c. motor to a load of the low voltage system as a low voltage system power source, wherein a voltage of the low voltage system is made a specified value of between about xc2xd to about xc2xc of the voltage of the high voltage system, and the inverter is controlled so that a current distortion phenomenon is caused in the generated voltage waveform at least at the time of motor generation.
A voltage ratio of about 1:2 or about 1:4 indicates a voltage ratio in a range of from 1:2 to 1:4 even taking into consideration an error.